The present invention relates to a stencil printer selectively operable in a simplex print mode with a perforated master or in a duplex print mode with perforated masters.
Digital printing of the type using a thermosensitive stencil is a convenient printing method extensively used today. In this type of printing method, a thermal head having an array of fine heating elements contacts a thermosensitive stencil. While current is selectively fed to the heating elements in the form of pulses, the stencil is conveyed along a preselected path. As a result, the stencil is perforated by heat in accordance with image data and then cut at a preselected length to turn out a master. The master is wrapped around a drum implemented as a porous hollow cylinder. Ink is transferred from the drum to a paper via the perforations of the master so as to print an image on the paper.
The current trend in the stencil printers art is toward duplex printing, i.e., printing images on both sides of a paper in order to reduce the consumption of papers. It has been customary to effect duplex printing by feeding a paper from a paper feeding section to a printing section, printing an image on one side of the paper, turning the paper upside down, and again feeding the paper to the printing section in order to print an image on the other side of the paper. Such a conventional duplex printing process, however, has the following problems (1)-(3) left unsolved.
(1) Papers driven out and each carrying an image on one side thereof must be again stacked on the paper feeding section. In addition, the papers carrying images on one side thereof must be neatly positioned. Such manual work is time- and labor-consuming.
(2) The ink on the papers or printings is not sufficiently dry just after the printing operation. Therefore, when images are immediately printed on the other sides of the papers, conveyor rollers and a press roller pressed against the images existing on the papers smear or otherwise disfigure the images. It is therefore a common practice to print images on the other sides of the papers on the elapse of several hours or so. Particularly, when the images existing on one side of the papers include solid portions, the papers must be dried over a long period of time, even over to the next day. This is undesirable from the efficiency standpoint.
(3) Because each paper is passed through the printing section twice, the conventional duplex printing consumes twice longer period of time than simplex printing even in net duration.
To solve the above problems, Japanese Patent Laid-Open Publication Nos. 6-71996 and 6-135111, for example, each teaches a stencil printer including a pair of drums facing each other and pressing them against each other in order to produce a duplex printing in a single step. In this type of stencil printer, one of the two drums is angularly movable into contact with the other drum. This kind of scheme, however, brings about another problem that the drums produce noise on contacting each other during printing, and image quality is not stable due to irregular rotation.